DC092A - Demo Manual

DEMO MANUAL DC092
GaAsFET BIAS GENERATOR
LTC1550/LTC1551
Low Noise, Switched Capacitor,
Regulated Voltage Inverter
U
DESCRIPTIO
Demonstration circuit DC092 is a low noise, regulated
voltage inverter using the LTC®1550/LTC1551. The all
surface mount design demonstrates the small size and
compact layout possible with the LTC1550/LTC1551. It is
designed for applications such as bias voltage generators
for GaAs transmitter FETs in portable RF equipment and
cellular telephones. This demo board highlights the low
noise capabilities of the LTC1550/LTC1551, which uses
an internal linear post regulator to limit output ripple to
1mVP-P.
The LTC1550/LTC1551 provide a TTL compatible Shutdown pin that reduces the supply current typically to
0.2µA. The LTC1550/LTC1551 are identical except for the
polarity of the Shutdown pin. DC092-A uses the LTC15504.1CS8, which has an active low Shutdown pin (SHDN),
while DC092-B uses the LTC1551-4.1CS8 with an active
high Shutdown pin (SHDN). The LTC1550/LTC1551 operate with a supply voltage of 4.5V to 6.5V and produce a
fixed – 4.1V ±5% output. Normal operating quiescent
current is 4mA at no load condition.
Other members of the LTC1550/LTC1551 family contain
additional features such as adjustable output voltage,
lower minimum VCC voltage requirements and an opendrain output that indicates when the output voltage is in
regulation. Please refer to the LTC1550/LTC1551 data
sheet for more information.
, LTC and LT are registered trademarks of Linear Technology Corporation.
WW
U W
PERFOR A CE SU
ARY (– 30°C < TA < 85°C)
SYMBOL
VCC
IS
PARAMETER
Supply Voltage
Supply Current
CONDITIONS
IOUT = 0mA
IOUT = 5mA
IOUT = 10mA
Shutdown
BOARD SUFFIX
All
All
All
All
All
SHDN
Shutdown
LTC1550-4.1
LTC1551-4.1
A
B
VALUE
4.5V to 6.5V
4mA Typ
9mA Typ
14mA Typ
0.2µA Typ
TTL Low Level
TTL High Level
U W
U
TYPICAL PERFOR A CE CHARACTERISTICS A D BOARD PHOTOS
VOUT Output Noise and Ripple
Component Side
2mV/DIV
VIN = 5V
VOUT = – 4.1V
IOUT = 5mA
AC COUPLED
10µs/DIV
DC092 TA01
1
DEMO MANUAL DC092
GaAsFET BIAS GENERATOR
WW
U W
PERFOR A CE SU
SYMBOL
PARAMETER
fOSC
Internal Oscillator Frequency
VRIPPLE
Ripple Voltage
VOUT
Output Voltage
ARY
CONDITIONS
BOARD SUFFIX
VALUE
All
VCC = 4.5V, 0 < IOUT < 5mA
VCC = 5V, 0 < IOUT < 10mA
VCC = 6V, 0 < IOUT < 20mA
VCC = 6.5V, 0 < IOUT < 25mA
900kHz Typ
All
1mVP-P Typ
All
All
All
All
– 3.9V to – 4.3V
– 3.9V to – 4.3V
– 3.9V to – 4.3V
– 3.9V to – 4.3V
W
W
U
PACKAGE A D SCHE ATIC DIAGRA S
U1
LTC1550
LTC1551
SHUTDOWN
E3
VCC
4.5V TO 6V
+
E5
GND
C3
10µF
10V
1
2
1
JPR1
2
3
3
4
TOP VIEW
SHDN
SENSE
VCC
CPOUT
C1+
GND
VOUT
C1 –
8
7
6
C2
0.1µF
E4
5
E1
BEAD
VOUT
J1
BNC
C1
0.1µF
COUT
10µF
10V
C4
0.1µF
DC092 F01
+
E2
SHDN* 1
8
VCC 2
7
CPOUT
C1+ 3
6
GND
VOUT 4
5
C1 –
SENSE
S8 PACKAGE
8-LEAD PLASTIC SO
*SHDN FOR LTC1550-4.1
SHDN FOR LTC1551-4.1
LTC1550-4.1CS8
LTC1551-4.1CS8
Figure 1.DC092 Schematic (– 4.1V Generator)
PARTS LIST
REFERENCE
DESIGNATOR
C1, C2, C4
C3, COUT
E1
U1
2
QUANTITY
3
2
1
1
PART NUMBER
08055G104ZAT3S
595D106X0010A2T
ILB-1206-600
LTC1550-4.1CS8/
LTC1551-4.1CS8
DESCRIPTION
0.1µF 50V 20% Y5V Capacitor
10µF 10V 20% Tantalum Capacitor
600Ω ±25% Bead
Switched Capacitor Voltage Inverter
VENDOR
AVX
Sprague
Dale
LTC
TELEPHONE
(803) 946-0362
(207) 324-4140
(605) 665-9301
(408) 432-1900
DEMO MANUAL DC092
GaAsFET BIAS GENERATOR
U
OPERATIO
The DC092 demonstration board is intended for the evaluation of the LTC1550/LTC1551. Solid turret terminals are
included for easy connections to test equipment and external circuitry. The board schematic and device package
pinout are shown in Figure 1. Board DC092-A is assembled
with an LTC1550-4.1 which has an active low Shutdown pin;
board DC092-B uses the LTC1551-4.1 with active high shutdown logic. The two boards are otherwise identical. The circuit operates with input voltages of 4.5V to 6.5V. Output
voltage is – 4.1V ±5% with load currents from 0mA to
10mA. Component values, vendors and the PC board layout
are recommended by Linear Technology. In many cases the
layout can be copied exactly onto the system PCB.
output load to VOUT (E4) and GND (E2) terminals on the left
side of the board. Shunt connector at JPR1 should be in the
left position to turn on a DC092-A (LTC1550-4.1) board; the
right position to turn on a DC092-B (LTC1551-4.1). Noise
and output ripple measurements are made at the
female BNC socket (J1) on the demo board. Connect J1 to an
oscilloscope or spectrum analyzer input with a straight BNC
male-to-male adapter (Figure 5, item 4) or a short BNC maleto-male cable. For best results, please refer to the section
titled Ripple and Noise Measurement.
LTC1550/LTC1551 Operation
The LTC1550/LTC1551’s internal circuitry contains a
switched capacitor inverting charge pump and a linear post
regulator, as shown in Figure 2. The charge pump is driven
by an internal clock, which typically runs at 900kHz. Operating at this frequency helps to minimize the size of the
Hook-Up
Connect the input power supply to the VCC (E3) and GND
(E5) terminals on the right side of the board. Connect the
VCC
CPOUT
+
C2
VOUT
S1
Q1
+
S4
COUT
C1+
900kHz
CLK
C1
S2
C1–
OP AMP
+
–
S3
CHARGE
PUMP
LINEAR
REGULATOR
1.24V
SENSE
DC092 F02
Figure 2. Block Diagram of the LTC1550/LTC1551
3
DEMO MANUAL DC092
GaAsFET BIAS GENERATOR
U
OPERATIO
external components and places any noise out of the 400kHz
to 600kHz IF band that is commonly used by portable radio
frequency systems.
reject virtually all of the noise generated by the charge
pump circuitry, resulting in output noise below 1mVP-P
with a proper PCB design.
At the beginning of each clock cycle, capacitor C1 is charged
to VCC by S1 and S3. At the next clock edge, S1 and S3 open
and S2 and S4 close. This connects C1 + to ground and the
charge in C1 is transferred to C2 through S4, pulling CPOUT
to a negative voltage. As the charge pump continues to run,
CPOUT approaches a voltage equal to – VCC.
DCO92 Performance
The voltage at VOUT is regulated from CPOUT by the internal
linear regulator. A divided version of the output voltage at
SENSE is compared to the internal reference voltage by the
internal feedback amplifier. The output of the feedback
amplifier controls the gate of the series pass transistor Q1
in a negative feedback loop. As the output voltage changes
due to changes in output load current or input ripple voltage, the feedback loop adjusts the gate drive at Q1 to keep
the output voltage at a constant level. This loop is able to
Figure 3 shows the output spectrum over a range of 100kHz
to 10MHz with V CC = 5V, IOUT = 5mA. The fundamental
switching noise is less than 3µV and all related harmonics
quickly drop into the spectrum analyzer’s noise floor.
DC092 is a good example of layout that minimizes output
noise and ripple in an LTC1550/LTC1551 design. The output
ripple is typically below 1mV with output loads between
0mA and 10mA (see VOUT Output Noise and Ripple).
Figure 4 shows a plot of low frequency spot noise (1kHz to
100kHz). Throughout the audio range the noise is about 600
to 700nV/√Hz. The broad band output noise above 20kHz is
attenuated by the 10µF output capacitor.
90
10
80
70
1
50
µV/√Hz
10dBµV/DIV
60
40
30
0.1
20
10
0
– 10
0.1
1
FREQUENCY (MHz)
10
DC092 F03
Figure 3. Output Spectrum 0.1MHz to 10MHz
4
0.01
1k
10k
FREQUENCY (kHz)
100k
DC092 F04
Figure 4. Output Spot Noise 1kHz to 100kHz
DEMO MANUAL DC092
GaAsFET BIAS GENERATOR
U
OPERATIO
The DC092 circuit typically draws 4mA quiescent current
under no load conditions. Current draw with a 5mA load and
a 5V supply is typically 9mA. In shutdown, the output voltage
falls to 0V and the quiescent current drops to 0.2µA.
voltage. At least 4.7µF is required at the output to maintain
loop stability. For optimum output stability over temperature with minimum output voltage ripple, a value of 10µF or
greater is recommended.
Components
A small 0.1µF ceramic capacitor can be placed in parallel
with COUT to reduce the high frequency noise. LC or RC
filtering at the output will also effectively reduce the ripple
and noise further. The LTC1550/LTC1551’s high 900kHz
charge pump frequency allows effective output filtering
with small valued components. The DC092 uses a small
ferrite bead and a surface mount 0.1µF capacitor as an
output filter to bring output voltage ripple and noise to
below 1mVP-P (see Parts List).
The LTC1550/LTC1551 require only four external components to operate: two charge pump capacitors (C1, C2), an
input bypass capacitor (C3) and an output capacitor (COUT).
DC092 includes an additional LC output filter to further
reduce output noise and ripple.
The input bypass capacitor (C3) is very important for
proper operation. It provides most of the current when
charging the flying capacitor. Select a low ESR type and
mount as close to the LTC1550/LTC1551 as possible. Select C3’s value to be significantly larger than the flying capacitor (C1) to prevent excessive ripple at VCC when the
charge pump is heavily loaded. It is often convenient to use
the same type of capacitor for C3 and COUT. The Sprague
595D106X0010A2T (10µF, 10V tantalum capacitor) is a
small surface mount device and makes a good choice for
C3 and COUT.
The charge pump capacitors, C1 and C2, are less critical
since their peak currents are limited by the switches inside
the LTC1550/LTC1551. Typical surface mount ceramic
capacitors with a value of 0.1µF are recommended for C1
and C2 (see Parts List).
Ripple and Noise Measurement
Accurately measuring output noise and ripple in high speed
switching circuitry like the LTC1550/LTC1551 can be challenging and error prone. The input voltage supply to the
demo board should be a low noise DC source, supplied either by a battery or a well regulated DC supply. Circuit and
test equipment grounds should be carefully connected to
minimize ground noise and ground loops. Due to the high
current paths in the LTC1550/LTC1551 internal charge
pump, minor potential differences between the tested circuit
and the measuring instrument (typically an oscilloscope or a
spectrum analyzer) can cause ground currents in the measuring probe and induce significant voltage errors in the
measurement.
The output capacitor (COUT) supplies current to the load during alternate charge pump cycles and sets the output ripple
5
DEMO MANUAL DC092
GaAsFET BIAS GENERATOR
U
OPERATIO
Figure 5 shows a typical 10X passive probe and accessories.
The use of the ground lead (item 2) and the retractable hook
tip (item 6) for measurement should be avoided. This is
because the unshielded probe body makes a remarkably
efficient antenna at the 900kHz switching frequency of the
LTC1550/LTC1551, introducing significant additional noise
(as well as the local all talk AM radio station signal) into the
measurement. A better choice is to attach a bayonet ground
clip (item 3) to the probe tip and take the measurement
directly across the output capacitor, COUT. A BNC-to-probe
tip adapter (item 5) can also be used to connect the probe to
the BNC socket (J1) on the demo board.
Similar results are obtained by using a 50Ω or 75Ω
coaxial BNC male-to-male cable to connect J1 on the demo
board to the oscilloscope’s BNC input. The length of the
cable should be as short as possible. Very good results are
obtained by connecting J1 on the demo board to the
oscilloscope’s BNC input through a straight BNC male-tomale adapter (item 4). Either of these connections provide a
good measurement to allow an accurate assessment of the
DC092’s circuit performance.
Very high quality measurements are obtained with
active probes designed specifically for the analyzer or oscilloscope used. Active probes maintain a high input impedance throughout their frequency range and minimize
disturbances to the tested circuit. They also provide accurate and consistent measurement results. The output spectrum and spot noise plots shown in Figures 3 and 4 were
taken with an HP41800A active probe and an HP4195A
spectrum analyzer. The drawback of this method is that the
active probe is more costly than a BNC connector or a passive probe. The active probe must also be properly matched
to the analyzer or oscilloscope to ensure accurate results.
2
3
4
1
5
ITEM
DESCRIPTION
1
Probe Body
2
Ground Lead
3
Bayonet Ground Clip
4
BNC Male-to-Male Adapter
5
BNC-to-Probe Tip Adapter
6
Retractable Hook Tip
6
Figure 5: Probe and BNC Accessories
6
DEMO MANUAL DC092
GaAsFET BIAS GENERATOR
W
U
PCB LAYOUT A D FIL
Component Side Silkscreen
Component Side
Component Side Solder Mask
Pastemask
Solder Side
Solder Side Solder Mask
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
7
DEMO MANUAL DC092
GaAsFET BIAS GENERATOR
U
PC FAB DRAWI G
2.100
D
D
D
D
B
C
1.450
A
D
D
D
D
NOTES:
1. ALL DIMENSIONS ARE IN INCHES
2. MATERIAL: FR4 OR EQUIVALENT EPOXY, 2 OZ COPPER CLAD
THICKNESS 0.061 ± 0.006 TOTAL OF 2 LAYERS
3. FINISH: ALL PLATED HOLES 0.001 MIN/0.0015 MAX COPPER PLATE
ELECTRODEPOSITED TIN-LEAD COMPOSITION BEFORE REFLOW,
SOLDER MASK OVER BARE COPPER (SMOBC)
4. SOLDER MASK: BOTH SIDES USING GREEN PC-401 OR EQUIVALENT
5. SILKSCREEN: WHITE NONCONDUCTIVE INK BOTH SIDES
6. SCORING:
0.017
SYMBOL
DIAMETER
NUMBER
OF HOLES
A
0.025
1
B
0.040
3
C
0.050
1
D
0.095
8
TOTAL HOLES
13
DC092 FAB
8
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417 ● (408) 432-1900
FAX: (408) 434-0507● TELEX: 499-3977 ● www.linear-tech.com
dc092f LT/TP 0497 500 • PRINTED IN USA
 LINEAR TECHNOLOGY CORPORATION 1997